This invention relates to a broadband radome for use in a high temperature environment such as those imposed by high supersonic and hypersonic speeds by providing a radome wall utilizing one of a group of ceramic materials consisting of silicon nitride and barium-aluminum silicate to form a core layer and a skin layer. The invention further provides a high temperature adhesive and a base connector for transitional support of a ceramic radome for structural adequacy on a body of an aircraft or the like for use in a high temperature environment.
Broadband radomes for us in the high temperature environment generated by high supersonic and hypersonic speeds must possess structural adequacy to withstand severe thermal and aerodynamic loads as well as erosion effects due to rain and dust. The materials and production processes to produce radomes for such an environment must allow consistent accurate production while achieving high electrical performance with minimal error over broad bandwidths and/or large antenna scan angles. Prior attempts to provide a radome to meet the demands for such high temperature environments have not successfully solved the problems due to several shortcomings.
A solid wall ceramic radome such as employed in the Sparrow and Phoenix missiles has limited rain erosion capabilities as well as a limited bandwidth capability. The conventional A-type and C-type sandwich constructions for broadband radomes using high temperature materials have a limited structural adequacy to withstand thermal gradients because of high tensile stresses developed in the radome wall.
In U.S. Pat. No. 3,292,544 there is disclosed a radome wall consisting of thin inner and outer layers with a dielectric constant of 8-12 bonded to a thick central core with a dielectric constant of 2-5. Alumina, mullite or calcined sillimanite is used to make the walls of the radome. Such a radome is designed for use at a discrete frequency defined by controlling the inner and outer wall layer thicknesses so as not to exceed 0.06 of the full wavelength of the energy to be transmitted. The transmission of electromagnetic energy is within a relatively narrow bandwidth as compared with a bandwidth in excess of 4 octaves which can be transmitted by a radome embodying the features of the present invention. The inorganic refractory oxide ceramic materials and the three-layered construction of the radome wall provide only limited resistance to thermal gradients when compared with the radome of the present invention. This is because temperature differences between the outer and inner wall layers develop high tensile stresses in the lower temperature thin-walled layer and a high transverse tensile stress on the central core and on the bonds between the core and the inner and outer layers. However, the inorganic refractory oxide materials have only limited resistance to these tensile stresses.
The radome wall is highly susceptible to local variations in density, strength and electrical properties due to a non-uniform chemical composition of the bonded refractory because about 10% bonding materials such as clay, crystal growth inhibitors and mineralizers are admixed with the refractory oxide. Any such local variation produces a substantial variation to the electrical performance as well as variations in strength from one radome to the next. These variations bring about increased boresight error, the requirement for expensive prescription grinding of each radome and an increased rejection rate of finished radomes because of unacceptable electrical performance or low strength. The present invention eliminates dependency on bonding materials and utilizes a two-layered homogeneous ceramic material, thus avoiding the limitations of known refractory oxide materials for a radome wall.
Precision control of the inner and outer layer thicknesses is essential for a broadband application because of the relatively high dielectric constant of these radome layers. Only expensive prescription grinding of one or both of the inner and outer layers as a final step in the radome construction can be carried out to provide precise control of uniform electrical performance. In contrast to this, the present invention provides that the manufacturing tolerances are far less critical because lower dielectric constant materials are used, but without sacrificing broadband electrical performance, strength or incurring the substantial expense for prescription grinding.
A slurry or suspension of the inorganic refractory oxide is initially formed to manufacture the known radome walls. The slurry is then molded, dried and fired which causes a shrinkage of the radome of between 1.3% to about 10%-12%. The firing process increases the tendency for radome wall cracks and diminishes the ability for accurate control of critical radome dimensions. Moreover, in the manufacturing operations, organic hollow particles of a volatilizable resin are admixed with the bonding material and inorganic refractory when the slurry or suspension is formed. The use of such hollow resinous particles is necessary to vary the density of the material but is an unacceptable addition to the material used to form the radome wall of the present invention.